1. Field of the Invention
The present invention relates to a motor control device that detects position of the rotor of a motor and then controls the position and the speed thereof.
2. Description of the Related Art
In order to run a motor, the motor control device usually detects the rotational position of the rotor by a rotational position sensing device and energizes the stator winding thereof with the current which controls the position or velocity of the rotor thereof at a desired value. FIGS. 9 and 10 illustrate conventional motor control. The power transistor 5 shown in FIGS. 9 and 10 is illustrated in FIG. 11. The rotor 3 is illustrated in FIG. 12. Referring to FIG. 11, the power transistor 5 is equivalent to the so-called transistor inverter. In this example, the inverter is formed of six transistors 22 and six diodes 23, and a power source 21. The operation and function of the transistors 22 and the diodes 23 are well known. Hence the explanation will be omitted here. The motor is controlled by adjusting the current flowing through the rotor windings 28 of a motor. In order to control of the motor by the power transistor 5, the voltage between the terminals of each rotor winding 28 is controlled by switching respective transistor 22 so that a current at a desired value flows through each rotor winding 28.
FIG. 12 is a cross-sectional view showing the rotor 3. In the rotor 3, a certain number of discs are laminated together and mounted on the shaft 19. Each disc has slits 18 on the surface thereof and a certain thickness. The magnetic reluctance of the rotor 3 depends on the rotational position because of the present of the slits 18. FIG. 12 illustrates a 4-phase rotor. Returning to FIG. 9, the motor 2 mainly consists of a rotor 3, a stator 4, and rotational position sensing system 9. The rotational position sensing system 9 is position detection system that can measure the incremental position of the rotor 3. The rotor 4 connects to the power transistor 5 with three-phase power lines through which the motor 2 is energized by the power transistor 5. The motor control system 20 couples the power transistor 5 and the rotational position sensing system 9 with communication lines. In response to the incremental positional data POS2 of the rotor 3 from the rotational position sensing system 9, the motor control system 20 controls the position and velocity by system of a well-known technique, thus calculating a desired current value to energize the motor 2. After calculation of the current value, the motor control system 20 commands the power transistor 5 to supply the current command value Iref The power transistor 5 is controlled by the command value to flow the current Iref through the stator winding 28 in the stator 4 of the motor 2. Thus, the motor 2 generates a rotational force with the current Ip and thus rotates the rotor 3.
Referring to FIG. 10, the motor 2 consists of a rotor 3, a stator 4, rotational position sensing system 9, and origin position detecting system 12. The origin position detecting system 12 is position detecting system that can measure the origin position in one revolution of the rotor 3. The origin position detecting system 12 is coupled to the motor control system 20 by way of communication lines, like the rotational position sensing system 9. The motor control system 20 simultaneously receives data POS2 on incremental position the rotor 3 from the rotational position sensing system 9 as well as data POS 3 on origin poisition of the rotor 3 from the origin position detecting system 13. The motor control system 20 calculates data on position of the rotor 3 based on data POS2 and POS3 and performs the above-mentioned control so as to supply the current Ip to the motor based on the resultant data.
As described earlier with FIGS. 9 and 10, the position of the rotor can be detected by using either the incremental positional data POS2 of the rotor 3 or positional data by obtained combining the incremental positional data POS2 with the origin positional data POS3 of the rotor 3. This allows the motor to be precisely controlled. However, according to the conventional positional detection, the position of the rotor 3 cannot be detected just before the motor 2 is driven by turning on power. The position of the rotor 3 just before the motor is driven is hereinafter referred to as an initial position of the rotor 3. Since the rotational position sensing system 9 senses the incremental position of the rotor 3 when the teeth of a gear, for example, attached on the rotor 3 cross the detection plane thereof, the positional detection cannot be performed when the rotor 3 does not rotate. Moreover, the origin position detecting system 13 detects only one channel grooved in the disc attached on the rotor 3 according to the above-motioned detection method. Hence, the positinal detection cannot be performed if the rotor 3 rotates at least one turn. As described above, the initial position of the rotor 3 cannot be detected by means of only the rotational position sensing system 9 or both the rotational position sensing means 9 and the origin position detecting system 13.
A conventionally-improved positional position sensing device that detects the initial position of the rotor 3 will be described below with reference to FIGS. 11 and 13. FIG. 13 shows the relation between terminal voltage V (y-axis) and time (x-axis) and the relation between detection current Im (y-axis) and time (x-axis). In FIG. 11, the current metering system 6 consists of a current detector 24, an insulating resistor 25, an A/D converter 26, and rotational position arithmetic system 27. In FIG. 13, V represents a terminal voltage across the stator winding 28. That is, the stator winding 28 is formed of three phase coils. The terminal voltage between arbitrary two phase coils is V. The detection current Im is a current flowing through one of the two phase coils between which the voltage V is applied. Hence, the relation between V and Im is represented as illustrated in FIG. 11. As described earlier, since the power transistor 5 can control the voltage between the terminals of the stator winding 28 by switching the transistors 22, the terminal voltage V having the waveform shown in FIG. 13 can be applied across the stator winding 28. In this case, the detection current Im flowing through the stator winding 28 increases with a constant slope according to the inductance of the stator winding 28 and then begins to decrease when the terminal voltage V becomes 0. On the other hand, in the current metering system 6, the A/D converter 26 converts a current value draining through the insulating resistor 25 from the current detector 24 into digital data. The rotational position arithmetic system 27 receives data on current value and then calculates the gradient k of the detection current Im to time. Since k is a differential coefficient of detection current Im to time, k=dIm/dt (Eq. 1). The relation between gradient k of detection current Im and terminal voltage V is repressed by the equation V/2=L.times.k+R.times.Im (Eq. 2), where L is inductance and R is a resistance value per phase of the stator winding 28. Hence, the inductance can be obtained by the above-mentioned equation. The inductance of the stator winding 28 contains the mutual inductance between the rotor 3 and the stator winding 28, in addition to the self inductance. Since the mutual inductance varies by the polar position of the rotor 3, the inductance to be measured varies with the position of the rotor 3. Hence, the position of the rotor 3 can be decided by sequentially performing a certain calculation based on the inductance value. The terminal voltage V can be controlled as shown in FIG. 14 by switching the power transistor 5 shown in FIG. 11. In this case, there is the advantage of quickly converging the detection current Im. As described above, the position of the rotor 3 can be decided by means of the current metering system 6.
As described above, the position of the rotor 3 can be decided according to the prior art. However, the position of the rotor 3 to be decided corresponds to a change in inductance per period. FIG. 15 is a waveform showing changes in inductance within an electrical angle of 360.degree. of a conventional N-phase motor. As understood from the figure, There are changes in inductance for two periods within the electrical angle of 360.degree.. Hence, when the position of the rotor 3 is decided according to the inductance position detecting method, only the position for an electrical angle of 180.degree. can be measured. If the rotor 3 is formed of a permanent magnet, the rotational direction of the rotor 3 cannot be controlled because the polarity of the rotor 3 changes every electrical angle of 180.degree.. If a measured position of the rotor 3 differs from an actual position by an electrical angle of 180.degree., the rotor 3 reversely rotates to the direction by the command. As described above, the prior art can detect the initial position of a rotor and measures only the position of the rotor for an electrical angle of 180.degree.. Hence, there is the disadvantage in that where an opposite polarity comes on the rotor of a permanent magnet every electrical angle of 180.degree., the rotor may be reversely rotated.
According to the prior art, the position of the rotor 3 can be measured in the above-mentioned method. However, since the position of the rotor 3 is calculated by metering the detection current Im in active state, this method causes large detection errors. It is may be considered that such a method is unsuitable for precision control. The reason is that the measured value contains large noises upon current detection. As the number of revolutions increases, the errors upon inductance detection increase. Since the precision of inductance to be calculated worsens necessarily, precise control cannot be performed. There is the disadvantage in that the prior art described above is unsuitable for precise control because the initial position of the rotor cannot be detected or the detection error of initial position is large.